Characteristics of phosphate transport in isolated proximal tubule

The characteristics of inorganic phosphate transport in isolated perfused proximal tubules of the rabbit were examined using radioisotopic techniques. When tubules were perfused with an ultrafiltrate of rabbit serum, the mean lumen-to-bath flux of phosphate in the convoluted segment was 6.60 +/- 1.41 (SE) pmol/mm-min with a simultaneous back-to-lumen flux of 0.45 +/- 0.08. In the straight portion of the proximal tubule, the lumen-to-bath flux was significantly lower (P less than 0.01) at 2.22 +/- 0.48 pmol/min-min with a bath-to-lumen flux of 0.31 +/- 0.05. The lumen-to-bath flux was not affected by increases in the intraluminal phosphate concentration from 2.00 +/- 0.19 to 3.12 +/- 0.34 mM or by the isohydric replacement of bicarbonate in the ambient fluids with chloride. However, phosphate absorption was completely inhibited by ouabain 10(-5) M in the bath. These data indicate that phosphate absorption in these segments occurs by a mechanism other than independent diffusion and is saturated at phosphate concentrations characteristic of normal glomerular filtrate. There is no evidence for significant phosphate transport from bath to lumen.